How To Cast Thicker Items?

Hello Ceramic Folks, I am interested in making duplicate copies of an item made from ceramic. This item would be basically rectangular in shape, about 31/2 “ long, about 1” wide, ¾ “ thick, with a ¼ “ centered hole running its length. I keep running across things on slip casting but that seems more appropriate for thinner objects like vases and such. I know little of ceramics, so if someone could direct me to some reference work on how I might go about such a project it would be appreciated. Thanks, Jack

Hello Ceramic Folks, I am interested in making duplicate copies of an item made from ceramic. This item would be basically rectangular in shape, about 31/2 “ long, about 1” wide, ¾ “ thick, with a ¼ “ centered hole running its length. I keep running across things on slip casting but that seems more appropriate for thinner objects like vases and such. I know little of ceramics, so if someone could direct me to some reference work on how I might go about such a project it would be appreciated. Thanks, Jack

IMHO the thickness shouldn't be a major problem. However de-moulding the piece might be a major issue.

I've cast items with vertical sides, and the piece de-moulds reluctantly, but without major difficulty. Mainlybecause the clay shrinks away from the outer walls of the mould, giving a little wriggle-room.

On the other hand, the clay will shrink onto the part of the mould modelling your central hole. Permanently.

You might get away with a very tapered hole, but I doubt it.

Regards, Peter

At this point you have to think about the accuracy you require, and the trouble you are willing to go to. Most of the ideas I've seen are from the technical-ceramic rather than the pottery world.

A direct route is simply to drill the hole. Either in the greenware, or the fired ceramic.

I've read of people having de-moulding problems pre-casting a layer of paper-pulp in the mould as a release agent. But this was because they were casting with difficult materials (alumina, zirconia, ...)rather than addressing shrinking-on problems.

In the past I've toyed with the idea of casting a sacrificial rod into the piece. Using a material that eitherburns away in the firing, or can be crumbled away after the firing. Our colleagues in glass-making and metal 'sand' casting have been round this route. Not that I found anything that obviously applied to pottery. - The world of casting-sand is a black art, often using exotic additives only available in industrial quantities.- Glass artists tend to use things which crumble on firing at their temperatures. From a distant memory things like plaster-of-paris with additives (e.g. sawdust?).

You might get away with making the sacrificial rod out of something like paper-mache, or even a cardboard tube.Might want to water-proof the cardboard tube so that it doesn't loose strength when it gets wet. If you can source tubes of reproducible size (waxed paper drinking straws?) you could have a recess in the mould to hold the straw.

Finally the thicker the casting the more time it takes, both to cast and the dry. Eventually you might want to considerusing some sort of paper-slip, with just enough paper-pulp to 'wick' the moisture. Might well reduce warping as well.